RHS 90×50×3.6 vs CF RHS 90×50×4
RHS 90×50×3.6 is an RHS Hot-finished section from the UK Blue Book (SCI P363); CF RHS 90×50×4 is the closest equivalent in the UK Blue Book (SCI P363). CF RHS 90×50×4 carries more bending: 28.8 cm³ of plastic modulus against 27.2 cm³, 5.6% more. RHS 90×50×3.6 is the lighter of the two at 7.4 kg/m against 7.97 kg/m, a saving of 7.2% on steel weight. Both are about 90 mm deep, so either fits the same construction zone.
| Property | RHS 90×50×3.6 RHS Hot-finished | CF RHS 90×50×4 RHS Cold-formed | Difference |
|---|---|---|---|
| Mass | 7.4 kg/m | 8 kg/m | -7.2% |
| Depth | 90 mm | 90 mm | same |
| Width | 50 mm | 50 mm | same |
| Web thickness | 0 mm | 0 mm | same |
| Flange thickness | 0 mm | 0 mm | same |
| Area | 9.4 cm² | 10.1 cm² | -6.7% |
| Second moment, major (Iy) | 98.3 cm⁴ | 103 cm⁴ | -4.6% |
| Plastic modulus, major (Wpl,y) | 27.2 cm³ | 28.8 cm³ | -5.6% |
| Elastic modulus, major (Wel,y) | 21.8 cm³ | 22.8 cm³ | -4.4% |
| Second moment, minor (Iz) | 38.7 cm⁴ | 40.7 cm⁴ | -4.9% |
| Plastic modulus, minor (Wpl,z) | 18 cm³ | 19.1 cm³ | -5.8% |
| Radius of gyration, minor (iz) | 2.03 cm | 2 cm | +1.5% |
| Torsion constant (It) | 89.4 cm⁴ | 97.7 cm⁴ | -8.5% |
RHS 90×50×3.6 to BS EN 10210-2:2006; CF RHS 90×50×4 to BS EN 10219-2:2006. Difference is expressed relative to CF RHS 90×50×4.
How to choose between them
On stiffness, CF RHS 90×50×4 has the larger second moment of area: 103 cm⁴ against 98.3 cm⁴, a difference of 4.6%. This matters more often than the strength comparison, because on normal floor spans deflection rather than bending capacity sets the beam size - so if the two are close on modulus but apart on inertia, the stiffer one is usually the better answer even when it is not the stronger one.
Measured as bending capacity per kilogram, the two are equally efficient (3.7 against 3.6 cm³ per kg/m), so on a pure steel-tonnage basis there is nothing to choose between them. The decision comes down to availability, connection detailing and what the rest of the frame uses.
In short, CF RHS 90×50×4 is the stronger section and RHS 90×50×3.6 is the lighter one; where those are the same section, it is the clear choice, and where they are not, decide on whether your design is governed by capacity or by tonnage.